ADS7804P

® ADS 780 4 ADS7804 DEMO BOARD AVAILABLE ADS 780 4 12-Bit 10µs Sampling CMOS ANALOG-to-DIGITAL CONVERTER FEATURES q q q q q q q q q q q q 100kHz min SAMPLING RATE STANDARD ±10V INPUT RANGE 72dB min SINAD WITH 45kHz INPUT ±0.45 LSB max INL DNL: 12 Bits “No Missing Codes” SINGLE +5V SUPPLY OPERATION PIN-COMPATIBLE WITH 16-BIT ADS7805 USES INTERNAL OR EXTERNAL REFERENCE COMPLETE WITH S/H, REF, CLOCK, ETC. FULL PARALLEL DATA OUTPUT 100mW max POWER DISSIPATION 28-PIN 0.3" PLASTIC DIP AND SOIC DESCRIPTION The ADS7804 is a complete 12-bit sampling A/D using state-of-the-art CMOS structures. It contains a complete 12-bit, capacitor-based, SAR A/D with S/H, reference, clock, interface for microprocessor use, and three-state output drivers. The ADS7804 is specified at a 100kHz sampling rate, and guaranteed over the full temperature range. Lasertrimmed scaling resistors provide an industrystandard ±10V input range, while the innovative design allows operation from a single +5V supply, with power dissipation under 100mW. The 28-pin ADS7804 is available in a plastic 0.3" DIP and in an SOIC, both fully specified for operation over the industrial –40°C to +85°C range. Clock Successive Approximation Register and Control Logic R/C CS BYTE BUSY CDAC 20kΩ ±10V Input 10kΩ 4kΩ Comparator CAP Buffer 4kΩ REF Internal +2.5V Ref Output Latches and Three State Drivers Three State Parallel Data Bus International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 • Twx: 910-952-1111 Internet: http://www.burr-brown.com/ • FAXLine: (800) 548-6133 (US/Canada Only) • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 © 1992 Burr-Brown Corporation PDS-1156C Printed in U.S.A. February, 1996 SPECIFICATIONS ELECTRICAL At TA = –40°C to +85°C, fS = 100kHz, and VDIG = VANA = +5V, using internal reference, unless otherwise specified. ADS7804P, U PARAMETER RESOLUTION ANALOG INPUT Voltage Ranges Impedance Capacitance THROUGHPUT SPEED Conversion Time Complete Cycle Throughput Rate DC ACCURACY Integral Linearity Error Differential Linearity Error No Missing Codes Transition Noise(2) Full Scale Error(3,4) Full Scale Error Drift Full Scale Error(3,4) Full Scale Error Drift Bipolar Zero Error(3) Bipolar Zero Error Drift Power Supply Sensitivity (VDIG = VANA = VD) AC ACCURACY Spurious-Free Dynamic Range Total Harmonic Distortion Signal-to-(Noise+Distortion) Signal-to-Noise Full-Power Bandwidth(6) SAMPLING DYNAMICS Aperture Delay Aperture Jitter Transient Response Overvoltage Recovery(7) REFERENCE Internal Reference Voltage Internal Reference Source Current (Must use external buffer.) Internal Reference Drift External Reference Voltage Range for Specified Linearity External Reference Current Drain DIGITAL INPUTS Logic Levels VIL VIH IIL IIH DIGITAL OUTPUTS Data Format Data Coding VOL VOH Leakage Current Output Capacitance DIGITAL TIMING Bus Access Time Bus Relinquish Time ±10V 23 35 5.7 Acquire and Convert 100 ±0.9 ±0.9 Guaranteed 0.1 ±7 Ext. 2.5000V Ref Ext. 2.5000V Ref ±2 ±2 +4.75V < VD < +5.25V ±0.5 ±0.5 ±10 * ±0.5 * * * ±5 * ±10 ±0.25 ±0.25 8 10 * ±0.45 ±0.45 CONDITIONS MIN TYP MAX 12 MIN ADS7804PB, UB TYP MAX * UNITS Bits * * * * * * V kΩ pF µs µs kHz LSB(1) LSB Bits LSB % ppm/°C % ppm/°C mV ppm/°C LSB fIN = fIN = fIN = fIN = 45kHz 45kHz 45kHz 45kHz 80 –80 70 70 250 40 Sufficient to meet AC specs 2 150 2.48 2.5 1 8 2.5 2.52 * * 72 72 * * * * * * * * * dB(5) dB dB dB kHz ns µs ns V µA ppm/°C V µA FS Step 2.3 Ext. 2.5000V Ref 2.7 100 * * * * –0.3 +2.0 +0.8 VD +0.3V ±10 ±10 * * * * * * V V µA µA ISINK = 1.6mA ISOURCE = 500µA High-Z State, VOUT = 0V to VDIG High-Z State +4 Parallel 12 bits Binary Two’s Complement +0.4 * ±5 15 * * 15 V V µA pF 83 83 * * ns ns The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. ® ADS7804 2 SPECIFICATIONS (CONT) ELECTRICAL At TA = –40°C to +85°C, fS = 100kHz, and VDIG = VANA = +5V, using internal reference, unless otherwise specified. ADS7804P, U PARAMETER POWER SUPPLIES Specified Performance VDIG VANA +IDIG +IANA Power Dissipation TEMPERATURE RANGE Specified Performance Derated Performance Storage Thermal Resistance (θJA) Plastic DIP SOIC CONDITIONS MIN TYP MAX MIN ADS7804PB, UB TYP MAX UNITS Must be ≤ VANA +4.75 +4.75 +5 +5 0.3 16 +5.25 +5.25 * * * * * * * * V V mA mA mW °C °C °C °C/W °C/W fS = 100kHz –40 –55 –65 75 75 100 +85 +125 +150 * * • * * * * * • NOTES: (1) LSB means Least Significant Bit. For the 12-bit, ±10V input ADS7804, one LSB is 4.88mV. (2) Typical rms noise at worst case transitions and temperatures. (3) As measured with fixed resistors shown in Figure 4. Adjustable to zero with external potentiometer. (4) Full scale error is the worst case of –Full Scale or +Full Scale untrimmed deviation from ideal first and last code transitions, divided by the transition voltage (not divided by the full-scale range) and includes the effect of offset error. (5) All specifications in dB are referred to a full-scale ±10V input. (6) Full-Power Bandwidth defined as Full-Scale input frequency at which Signal-to-(Noise + Distortion) degrades to 60dB, or 10 bits of accuracy. (7) Recovers to specified performance after 2 x FS input overvoltage. ABSOLUTE MAXIMUM RATINGS Analog Inputs: VIN ............................................................................. ±25V CAP ................................... +VANA +0.3V to AGND2 –0.3V REF .......................................... Indefinite Short to AGND2 Momentary Short to VANA Ground Voltage Differences: DGND, AGND1, AGND2 ................. ±0.3V VANA ....................................................................................................... 7V VDIG to VANA ..................................................................................... +0.3V VDIG ....................................................................................................... 7V Digital Inputs ........................................................... –0.3V to +VDIG +0.3V Maximum Junction Temperature ................................................... +165°C Internal Power Dissipation ............................................................. 825mW Lead Temperature (soldering, 10s) ............................................... +300°C ELECTROSTATIC DISCHARGE SENSITIVITY Electrostatic discharge can cause damage ranging from performance degradation to complete device failure. BurrBrown Corporation recommends that all integrated circuits be handled and stored using appropriate ESD protection methods. PACKAGE INFORMATION PRODUCT ADS7804P ADS7804PB ADS7804U ADS7804UB PACKAGE Plastic DIP Plastic DIP SOIC SOIC PACKAGE DRAWING NUMBER(1) 246 246 217 217 NOTE: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. ORDERING INFORMATION MINIMUM SIGNAL-TO(NOISE + DISTORTION) RATIO (dB) 70 72 70 72 PRODUCT ADS7804P ADS7804PB ADS7804U ADS7804UB MAXIMUM LINEARITY ERROR (LSB) ±0.9 ±0.45 ±0.9 ±0.45 SPECIFICATION TEMPERATURE RANGE –40°C –40°C –40°C –40°C to to to to +85°C +85°C +85°C +85°C PACKAGE Plastic DIP Plastic DIP SOIC SOIC ® 3 ADS7804 PIN # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 NAME VIN AGND1 REF CAP AGND2 D15 (MSB) D14 D13 D12 D11 D10 D9 D8 DGND D7 D6 D5 D4 D3 D2 D1 D0 (LSB) BYTE R/C CS BUSY VANA VDIG DIGITAL I/O DESCRIPTION Analog Input. See Figure 7. Analog Ground. Used internally as ground reference point. Reference Input/Output. 2.2µF tantalum capacitor to ground. Reference Buffer Capacitor. 2.2µF tantalum capacitor to ground. Analog Ground. O O O O O O O O Data Bit 11. Most Significant Bit (MSB) of conversion results. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 10. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 9. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 8. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 7. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 6. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 5. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 4. Hi-Z state when CS is HIGH, or when R/C is LOW. Digital Ground. O O O O O O O O I I I O Data Bit 3. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 2. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 1. Hi-Z state when CS is HIGH, or when R/C is LOW. Data Bit 0. Lease Significant Bit (LSB) of conversion results. Hi-Z state when CS is HIGH, or when R/C is LOW. LOW when CS LOW, R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW. LOW when CS LOW, R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW. LOW when CS LOW, R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW. LOW when CS LOW, R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW. Selects 8 most significant bits (LOW) or 8 least significant bits (HIGH). With CS LOW and BUSY HIGH, a Falling Edge on R/C Initiates a New Conversion. With CS LOW, a rising edge on R/C enables the parallel output. Internally OR’d with R/C. If R/C LOW, a falling edge on CS initiates a new conversion. At the start of a conversion, BUSY goes LOW and stays LOW until the conversion is completed and the digital outputs have been updated. Analog Supply Input. Nominally +5V. Decouple to ground with 0.1µF ceramic and 10µF tantalum capacitors. Digital Supply Input. Nominally +5V. Connect directly to pin 27. Must be ≤ VANA. TABLE I. Pin Assignments. PIN CONFIGURATION CHARACTERIZATION CURVES 28 VDIG 27 VANA 26 BUSY 25 CS 24 R/C 23 BYTE 22 DZ VIN AGND1 REF CAP AGND2 D11 (MSB) D10 D9 D8 1 2 3 4 5 6 7 ADS7804 8 9 Call factory for updated data sheet which includes characterization curves. 21 DZ 20 DZ 19 DZ 18 D0 (LSB) 17 D1 16 D2 15 D3 D7 10 D6 11 D5 12 D4 13 DGND 14 ® ADS7804 4 BASIC OPERATION Figure 1 shows a basic circuit to operate the ADS7804 with a full parallel data output. Taking R/C (pin 24) LOW for a minimum of 40ns (6µs max) will initiate a conversion. BUSY (pin 26) will go LOW and stay LOW until the conversion is completed and the output registers are updated. Data will be output in Binary Two’s Complement with the MSB on pin 6. BUSY going HIGH can be used to latch the data. All convert commands will be ignored while BUSY is LOW. The ADS7804 will begin tracking the input signal at the end of the conversion. Allowing 10µs between convert commands assures accurate acquisition of a new signal. The offset and gain are adjusted internally to allow external trimming with a single supply. The external resistors compensate for this adjustment and can be left out if the offset and gain will be corrected in software (refer to the Calibration section). Table II for a summary of CS, R/C, and BUSY states and Figures 3 through 5 for timing diagrams. CS and R/C are internally OR’d and level triggered. There is not a requirement which input goes LOW first when initiating a conversion. If, however, it is critical that CS or R/C initiates conversion ‘n’, be sure the less critical input is LOW at least 10ns prior to the initiating input. To reduce the number of control pins, CS can be tied LOW using R/C to control the read and convert modes. This will have no effect when using the internal data clock in the serial output mode. However, the parallel output will become active whenever R/C goes HIGH. Refer to the Reading Data section. CS 1 ↓ 0 0 ↓ ↓ 0 0 R/C X 0 ↓ 1 1 1 ↑ 0 BUSY X 1 1 ↑ 1 0 0 ↑ OPERATION None. Databus is in Hi-Z state. Initiates conversion “n”. Databus remains in Hi-Z state. Initiates conversion “n”. Databus enters Hi-Z state. Conversion “n” completed. Valid data from conversion “n” on the databus. Enables databus with valid data from conversion “n”. Enables databus with valid data from conversion “n-1”(1). Conversion n in process. Enables databus with valid data from conversion “n-1”(1). Conversion “n” in process. New conversion initiated without acquisition of a new signal. Data will be invalid. CS and/or R/C must be HIGH when BUSY goes HIGH. New convert commands ignored. Conversion “n” in process. STARTING A CONVERSION The combination of CS (pin 25) and R/C (pin 24) LOW for a minimum of 40ns immediately puts the sample/hold of the ADS7804 in the hold state and starts conversion ‘n’. BUSY (pin 26) will go LOW and stay LOW until conversion ‘n’ is completed and the internal output register has been updated. All new convert commands during BUSY LOW will be ignored. CS and/or R/C must go HIGH before BUSY goes HIGH or a new conversion will be initiated without sufficient time to acquire a new signal. The ADS7804 will begin tracking the input signal at the end of the conversion. Allowing 10µs between convert commands assures accurate acquisition of a new signal. Refer to X X 0 NOTE: (1) See Figures 2 and 3 for constraints on data valid from conversion “n-1”. Table II. Control Line Functions for “Read” and “Convert”. 200Ω 1 33.2kΩ + 2 2.2µF 3 2.2µF 28 27 26 25 24 23 22 ADS7804 21 20 19 18 17 16 15 B0 (LSB) B1 B2 B3 B4 B5 B6 B7 40ns min 6µs max Convert Pulse + 0.1µF + +5V 10µF + 4 5 B15 (MSB) B14 B13 B12 B11 B10 B9 B8 6 7 8 9 10 11 12 13 14 FIGURE 1. Basic Operation. ® 5 ADS7804 READING DATA The ADS7804 outputs full or byte-reading parallel data in Binary Two’s Complement data output format. The parallel output will be active when R/C (pin 24) is HIGH and CS (pin 25) is LOW. Any other combination of CS and R/C will tri-state the parallel output. Valid conversion data can be read in a full parallel, 12-bit word or two 8-bit bytes on pins 6-13 and pins 15-22. BYTE (pin 23) can be toggled to read both bytes within one conversion cycle. Refer to Table III for ideal output codes and Figure 2 for bit locations relative to the state of BYTE. DIGITAL OUTPUT BINARY TWO’S COMPLEMENT DESCRIPTION Full Scale Range Least Significant Bit (LSB) +Full Scale (10V – 1LSB) Midscale One LSB below Midscale –Full Scale ANALOG INPUT ±10V 4.88mV BINARY CODE HEX CODE PARALLEL OUTPUT (During a Conversion) After conversion ‘n’ has been initiated, valid data from conversion ‘n-1’ can be read and will be valid up to 16µs after the start of conversion ‘n’. Do not attempt to read data from 16µs after the start of conversion ‘n’ until BUSY (pin 26) goes HIGH; this may result in reading invalid data. Refer to Table IV and Figures 3 and 5 for timing specifications. Note! For the best possible performance, data should not be read during a conversion. The switching noise of the asynchronous data transfer can cause digital feedthrough degrading the converter’s performance. The number of control lines can be reduced by tieing CS LOW while using R/C to initiate conversions and activate the output mode of the converter. See Figure 3. SYMBOL 9.99512V 0V –4.88mV –10V 0111 1111 1111 0000 0000 0000 1111 1111 1111 1000 0000 0000 7FF 000 FFF 800 t6 t7 t8 t9 t10 t11 t7 + t6 t12 t13 t14 t1 t2 t3 t4 t5 DESCRIPTION Convert Pulse Width Data Valid Delay after R/C LOW BUSY Delay from R/C LOW BUSY LOW BUSY Delay after End of Conversion Aperture Delay Conversion Time Acquisition Time Bus Relinquish Time BUSY Delay after Data Valid Previous Data Valid after R/C LOW Throughput Time R/C to CS Setup Time Time Between Conversions Bus Access Time and BYTE Delay MIN TYP MAX UNITS 40 6000 8 65 8 220 40 7.6 8 2 10 50 35 200 7.4 9 10 10 10 83 10 83 ns µs ns µs ns ns µs µs ns ns µs µs ns µs ns Table III. Ideal Input Voltages and Output Codes. PARALLEL OUTPUT (After a Conversion) After conversion ‘n’ is completed and the output registers have been updated, BUSY (pin 26) will go HIGH. Valid data from conversion ‘n’ will be available on D11-D0 (pin 6-13 and 15-18 when BYTE is LOW). BUSY going HIGH can be used to latch the data. Refer to Table IV and Figures 3 and 5 for timing specifications. TABLE IV. Conversion Timing. BYTE LOW Bit 11 (MSB) Bit 10 Bit 9 Bit 8 6 7 ADS7804 8 9 21 LOW 20 LOW 19 LOW 18 Bit 0 (LSB) 17 Bit 1 16 Bit 2 15 Bit 3 Bit 1 Bit 0 (LSB) 8 9 23 22 LOW Bit 3 Bit 2 6 7 BYTE HIGH +5V 23 22 Bit 4 ADS7804 21 Bit 5 20 Bit 6 19 Bit 7 18 Bit 8 17 Bit 9 16 Bit 10 15 Bit 11 Bit 7 10 Bit 6 11 Bit 5 12 Bit 4 13 14 LOW 10 LOW 11 LOW 12 LOW 13 14 FIGURE 2. Bit Locations Relative to State of BYTE (pin 23). ® ADS7804 6 t1 R/C t13 t2 BUSY t3 t6 MODE Acquire Convert t7 Acquire t8 t5 Convert t4 DATA BUS Previous Data Valid t9 Hi-Z Previous Data Valid t11 Not Valid Data Valid t10 Hi-Z Data Valid FIGURE 3. Conversion Timing with Outputs Enabled after Conversion (CS Tied LOW.) t12 R/C t1 CS t12 t12 t12 t3 BUSY t4 t6 MODE Acquire Convert t7 Acquire DATA BUS Hi-Z State Data Valid t14 t9 Hi-Z State FIGURE 4. Using CS to Control Conversion and Read Timing. t12 R/C t12 CS BYTE Pins 6 - 13 Hi-Z High Byte t14 Low Byte t14 High Byte t9 Hi-Z Pins 15 - 22 Hi-Z Low Byte Hi-Z FIGURE 5. Using CS and BYTE to Control Data Bus. ® 7 ADS7804 INPUT RANGES The ADS7804 offers a standard ±10V input range. Figure 6 shows the necessary circuit connections for the ADS7804 with and without hardware trim. Offset and full scale error(1) specifications are tested and guaranteed with the fixed resistors shown in Figure 6b. Adjustments for offset and gain are described in the Calibration section of this data sheet. The offset and gain are adjusted internally to allow external trimming with a single supply. The external resistors compensate for this adjustment and can be left out if the offset and gain will be corrected in software (refer to the Calibration section). The nominal input impedance of 23kΩ results from the combination of the internal resistor network shown on the front page of the product data sheet and the external resistors. The input resistor divider network provides inherent overvoltage protection guaranteed to at least ±25V. The 1% resistors used for the external circuitry do not compromise the accuracy or drift of the converter. They have little influence relative to the internal resistors, and tighter tolerances are not required. NOTE: (1) Full scale error includes offset and gain errors measured at both +FS and –FS. SOFTWARE CALIBRATION To calibrate the offset and gain of the ADS7804 in software, no external resistors are required. See the No Calibration section for details on the effects of the external resistors. Refer to Table V for range of offset and gain errors with and without external resistors. NO CALIBRATION See Figure 6b for circuit connections. The external resistors shown in Figure 6b may not be necessary in some applications. These resistors provide compensation for an internal adjustment of the offset and gain which allows calibration with a single supply. The nominal transfer function of the ADS7804 will be bound by the shaded region seen in Figure 7 with a typical offset of –30mV and a typical gain error of –1.5%. Refer to Table V for range of offset and gain errors with and without external resistors. WITH EXTERNAL RESISTORS BPZ Gain Error –10 < BPZ < 10 –2 < BPZ < 2 –0.5 < error < 0.5 –0.25 < error < 0.25(1) WITHOUT EXTERNAL RESISTORS –45 < BPZ < 5 –8 < BPZ < 1 –0.6 < error < –0.55 –0.45 < error < –0.3(1) UNITS mV LSBs % of FSR CALIBRATION The ADS7804 can be trimmed in hardware or software. The offset should be trimmed before the gain since the offset directly affects the gain. To achieve optimum performance, several iterations may be required. HARDWARE CALIBRATION To calibrate the offset and gain of the ADS7804, install the proper resistors and potentiometers as shown in Figure 6a. The calibration range is ±15mV for the offset and ±60mV for the gain. NOTE: (1) High Grade. TABLE VII. Bipolar Offset and Gain Errors With and Without External Resistors. a) ±10V With Hardware Trim 200Ω ±10V 33.2kΩ +5V 2.2µF + b) ±10V Without Hardware Trim 200Ω 1 VIN ±10V 1 VIN 2 AGND1 33.2kΩ 2.2µF + 2 AGND1 3 REF 3 REF Offset 50kΩ 50kΩ Gain 576kΩ 4 2.2µF + 2.2µF CAP + 4 CAP 5 AGND2 5 AGND2 NOTE: Use 1% metal film resistors. FIGURE 6. Circuit Diagram With and Without External Resistors. ® ADS7804 8 Digital Output 7FF –10V –9.99983V –9.9998V –50mV –15mV 9.9997V 9.999815V +10V Analog Input Ideal Transfer Function With External Resistors Range of Transfer Function Without External Resistors 800 FIGURE 7. Full Scale Transfer Function. REFERENCE The ADS7804 can operate with its internal 2.5V reference or an external reference. By applying an external reference to pin 5, the internal reference can be bypassed. The reference voltage at REF is buffered internally with the output on CAP (pin 4). The internal reference has an 8 ppm/°C drift (typical) and accounts for approximately 20% of the full scale error (FSE = ±0.5% for low grade, ±0.25% for high grade). REF REF (pin 3) is an input for an external reference or the output for the internal 2.5V reference. A 2.2µF capacitor should be connected as close to the REF pin as possible. The capacitor and the output resistance of REF create a low pass filter to bandlimit noise on the reference. Using a smaller value capacitor will introduce more noise to the reference degrading the SNR and SINAD. The REF pin should not be used to drive external AC or DC loads. The range for the external reference is 2.3V to 2.7V and determines the actual LSB size. Increasing the reference voltage will increase the full scale range and the LSB size of the converter which can improve the SNR. ® CAP CAP (pin 4) is the output of the internal reference buffer. A 2.2µF capacitor should be placed as close to the CAP pin as possible to provide optimum switching currents for the CDAC throughout the conversion cycle and compensation for the output of the internal buffer. Using a capacitor any smaller than 1µF can cause the output buffer to oscillate and may not have sufficient charge for the CDAC. Capacitor values larger than 2.2µF will have little affect on improving performance. The output of the buffer is capable of driving up to 2mA of current to a DC load. DC loads requiring more than 2mA of current from the CAP pin will begin to degrade the linearity of the ADS7804. Using an external buffer will allow the internal reference to be used for larger DC loads and AC loads. Do not attempt to directly drive an AC load with the output voltage on CAP. This will cause performance degradation of the converter. 9 ADS7804 LAYOUT POWER For optimum performance, tie the analog and digital power pins to the same +5V power supply and tie the analog and digital grounds together. As noted in the electrical specifications, the ADS7804 uses 90% of its power for the analog circuitry. The ADS7804 should be considered as an analog component. The +5V power for the A/D should be separate from the +5V used for the system’s digital logic. Connecting VDIG (pin 28) directly to a digital supply can reduce converter performance due to switching noise from the digital logic. For best performance, the +5V supply can be produced from whatever analog supply is used for the rest of the analog signal conditioning. If +12V or +15V supplies are present, a simple +5V regulator can be used. Although it is not suggested, if the digital supply must be used to power the converter, be sure to properly filter the supply. Either using a filtered digital supply or a regulated analog supply, both VDIG and VANA should be tied to the same +5V source. GROUNDING Three ground pins are present on the ADS7804. DGND is the digital supply ground. AGND2 is the analog supply ground. AGND1 is the ground which all analog signals internal to the A/D are referenced. AGND1 is more susceptible to current induced voltage drops and must have the path of least resistance back to the power supply. All the ground pins of the A/D should be tied to the analog ground plane, separated from the system’s digital logic ground, to achieve optimum performance. Both analog and digital ground planes should be tied to the “system” ground as near to the power supplies as possible. This helps to prevent dynamic digital ground currents from modulating the analog ground through a common impedance to power ground. SIGNAL CONDITIONING The FET switches used for the sample hold on many CMOS A/D converters release a significant amount of charge injection which can cause the driving op amp to oscillate. The FET switch on the ADS7804, compared to the FET switches on other CMOS A/D converters, releases 5%-10% of the charge. There is also a resistive front end which attenuates any charge which is released. The end result is a minimal requirement for the anti-alias filter on the front end. Any op amp sufficient for the signal in an application will be sufficient to drive the ADS7804. The resistive front end of the ADS7804 also provides a guaranteed ±25V overvoltage protection. In most cases, this eliminates the need for external input protection circuitry. INTERMEDIATE LATCHES The ADS7804 does have tri-state outputs for the parallel port, but intermediate latches should be used if the bus will be active during conversions. If the bus is not active during conversion, the tri-state outputs can be used to isolate the A/D from other peripherals on the same bus. Tri-state outputs can also be used when the A/D is the only peripheral on the data bus. Intermediate latches are beneficial on any monolithic A/D converter. The ADS7804 has an internal LSB size of 610µV. Transients from fast switching signals on the parallel port, even when the A/D is tri-stated, can be coupled through the substrate to the analog circuitry causing degradation of converter performance. The effects of this phenomenon will be more obvious when using the pin-compatible ADS7805 or any of the other 16-bit converters in the ADS Family. This is due to the smaller internal LSB size of 38µV. APPLICATIONS Call factory for updated data sheet which includes standard DSP, microprocessor, and microcontroller interfaces. ® ADS7804 10